_Of the Brains and Nerves._
The brains are by all Physiologists esteemed the most mysterious organs
of life, being the fountain of the nerves, the organs of sensation, and
hence allowed by all to be the seat of the soul, or the _sanctum
sanctorum_ of the human understanding.
They are distinguished into the common or great brain called _cerebrum_;
the small part called _cerebellum_; and the _spinal marrow_, which
projects down the back-bone. The great brain is divided into two great
lobes, seated in the upper and fore part of the skull, of which it takes
in the greatest cavity. It is so exceedingly sensible, that the least
pressure puts an immediate stop to the sensation of the animal; and
which is generally termed a _apoplexy_.[2]
Footnote 2:
Hence in a fracture of the skull, or any other heavy contusion, where
the brain is compressed either by the fractured bone, or the
extravasated blood, the patient will remain in a state of
stupefaction, till either the fracture is reduced, or the extravasated
blood removed by trepannation; after which he immediately comes to
himself, as if suddenly revived from a trance.
This part of the brain, though it is so exquisitely sensible, seems to
be designed by nature as preparatory to the _cerebellum_, the immediate
root of the nerves.
The _cerebellum_ is the lesser, but the more material part of the
brains. It is seated in the hinder cavity of the skull, (which for the
sake of protecting this exquisite sensible organ, is remarkably strong
and firm) and from thence as the fountain of life itself, the nerves,
the instruments of sensation in the whole animal frame, take their
commencement: For whereas the other is so sensible, as that the least
pressure will cause a stupor; this is so far superior in point of
exquisite sensibility, that a pressure on that part, not only occasions
an apoplexy, but even stops life itself.
From this _cerebellum_ prolongates the marrow in the back-bone, called
_medulla spinalis_, which is but a continuation of the same, and is
equally sensible, and of the same substance with the former.
The whole brain, that is, the _great brain_, the _small brain_, _spinal
marrow_, together with all the nerves which spring from thence, are
covered with two coats, called the external stronger coat, _dura mater_;
and the thinner and inner coat, _pia mater_; which continue with each
nerve, even to its smallest ramification. The whole in a living subject
is in a continual motion peculiar to itself.[3]
Footnote 3:
This motion in the brain is not unlike the peristaltic motion; this is
an observation too seldom taken notice of by anatomists, though
evidently plain in living animals, and is of singular service in
physiological enquiries.
From the _cerebellum_, and _medulla spinalis_, all the nerves take their
commencement and origin. Ten pair spring forth from the part in the
skull; the principal are those which form the various senses, tasting,
smelling, hearing, and seeing, and that common to all, feeling. From the
_medulla spinalis_, or marrow in the back-bone, proceed thirty pair of
nerves, which spread themselves variously over the whole system.
All nerves have this in common with each other, namely, exquisite
sensibility; but in other respects they differ however widely;
particularly the nerves destined for some of the external senses.
It is from that common quality _feeling_, that they become the subtile
springs of life; and give an edge to all the animal functions.
There is nothing so plentifully, and nothing more curiously interwoven
with every other substance, as the nerves; and so amazingly quick is
their sensation, that the very instant a single nerve, even the smallest
and remotest in the system, is affected, the soul, or the common
_sensorium_, is sensible of the identical part so affected.
It has been asserted, that this amazing quick conveyance of the
sensation of the nerves, is owing to a subtile nervous fluid; but on a
rational reflection, and a just enquiry into the animal œconomy, it must
appear, that such conveyance of a fluid is inconsistent with probability
and experience: For, neither are the nerves tubulous, nor has there ever
been discovered any circulating humour in them. And secondly, is it
consistent with reason, that a conveyance of a fluid from the toe to the
finger, should be so momentarily quick, as is sometimes experienced in
the gout, and other the like ailments? That the quick transposition of
sensation of the nerves, is rather effected on a principle of
_vibration_, appears more probable, and will admit of a more convincing
experiment; which may be exemplified in the following manner: Suppose
(in the language of a seaman) a rope runs from the top-mast-head, or any
other part of the rigging, to the deck, and is kept tought and free from
any interception; then it will be evident, that the least touch at one
end will instantaneously be conveyed to the other end; in like manner
the nervous sensation may be communicated, perhaps not unlike
electricity; which similarly will in an instant be conveyed to any part,
by a direct communication, but equally subject to be intercepted.
Hence, a paralytic limb will lose, together with its irritable
sensibility, its circulation of fluids, and its nourishment; and recover
again when the pressure or interruption is ceased. Hence also, spasmodic
contractions will be affected at the irritation of some principal parts,
and from this instantaneous irritability, the motion of every part is
thought to obey the will.
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